import sys input = sys.stdin.readline T=int(input()) for tests in range(T): n,k=map(int,input().split()) A=list(map(int,input().split())) B=[a%k for a in A] B.sort() SUM=sum(B) if SUM%k!=0: print(-1) continue if n==k: x=B[0] flag=1 for i in range(n): if x!=B[i]: flag=0 break if flag: print(x) else: print(-1) continue X=[] for i in range(n): if B[i]!=0: X.append(B[i]) if len(X)==0: print(0) continue if len(X)==k: x=X[0] flag=1 for i in range(len(X)): if x!=X[i]: flag=0 break if flag: print(x) continue while len(X)<k+1: X.append(k) print(sum(X)//k)